1971 Super Lead 100W Janky Bias Reading

Arizona Joe

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So I have a SL 100 on the bench and after going through it I connected my bias meter to the two inside first then, the two outside 6550s. And I'm ony able to adjust the bias from 0 - 25 mA , acording to my chart Sovtek 6550s are 35W and my calculations tell me between 38mA (min) and 54mA (max). Anyone ever see this situation? Where I have the bias currently set now aprox 19ma -23mA it plays and sounds pretty good for no mods, which has me baffled beause you would expect the tubes (valves) to be starving and sputtering out. I've checked some of the voltages with no signal input and have 460v on the plate, 457 screen, -59.7mV on the control grid, 0v on the cathode. I suspected maybe one of the two 10uF 160v caps that set up the neg voltage for the bias circuit because I tested both with an ESR meter and the one tested a bit low. I'll have to wait til Monday to get a couple caps from the local amp supply house. Also, one difference I notice that in the control grid\bias node there is 220k resistors that split off to both sections. The schematic I have for the 1959 MKII with 6550 tubes shows what looks like 82k but, it appears this amp may have had the 220k from the beginning since they don't look like they have ever been replaced. Somethinfs robbing the bias current? Not sure what I'm missing, Thanks in advanced for any assistance.
 
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Pete Farrington

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Where I have the bias currently set now aprox 19ma -23mA it plays and sounds pretty good for no mods, which has me baffled beause you would expect the tubes (valves) to be starving and sputtering out.
I don’t expect that at all.
Fixed bias amps generally work and sound fine down to very low anode / cathode idle levels.
The spluttering type ‘zero crossing’ crossover distortion happens at literally near 0 idle anode current.
Just don’t expect hifi linearity at high power levels.
The amp is designed around 25W valves, so idle them at 18W max.
If lower idle dissipation sounds fine, then there’s no reason to query it, just enjoy the longer valve life.
 

Arizona Joe

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I don’t expect that at all.
Fixed bias amps generally work and sound fine down to very low anode / cathode idle levels.
The spluttering type ‘zero crossing’ crossover distortion happens at literally near 0 idle anode current.
Just don’t expect hifi linearity at high power levels.
The amp is designed around 25W valves, so idle them at 18W max.
If lower idle dissipation sounds fine, then there’s no reason to query it, just enjoy the longer valve life.
Thank you I will heed your advice. I was leaning that way, just wanted to make sure.
 


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